In ordinary rooms or in clean rooms, such as bio clean rooms, airborne particles, including microorganisms, are detected and recorded using particle detecting devices. See, for example, Japanese Unexamined Patent Application Publication Nos. 2008-225539 and 2011-83214, and N. Hasegawa, et al., Instantaneous Bioaerosol Detection Technology and Its Application, azbil Technical Review, 2-7, Yamatake Corporation, December 2009. An optical particle detecting device draws in air from the room wherein the device is installed, for example, and illuminates the drawn-in air with light. When there is a particle included within the air, a particle that is illuminated with light emits fluorescence or produces scattered light, enabling detection of the numbers, sizes, and the like, of particles included in the air.
Given this, an aspect of the present invention is to provide a particle detecting device and particle detecting method wherein particles can be detected accurately.